250+ TOP MCQs on Tautomerism and Answers

Organic Chemistry Multiple Choice Questions on “Tautomerism”.

1. How many enolisable hydrogens are there in the following compound?

a) 4
b) 5
c) 6
d) 7
Answer: a
Clarification: The hydrogen atom present on the carbon which is adjacent to the carbon attached to the functional group, and here it is 4.

2. How many tautomers can you draw for the following ketone?

a) 1
b) 2
c) 3
d) 4
Answer: b
Clarification: Two isomers can be drawn as shown below:

3. How many tautomers can you draw for the following ketone?

a) 1
b) 2
c) 3
d) 4
Answer: b
Clarification: Two isomers can be drawn as shown below.

4. Which of the following is a tautomer of phenol?
a)
b)
c)
d)
Answer: c
Clarification: According to the tautomerism the tautomer of phenol is c.

5. Which of the following compound exhibit tautomerism?

a) I
b) II
c) I and II
d) Neither I nor II
Answer: c
Clarification: For Keto-Enol tautomerism, keto form should have α-hydrogen.

6. In keto-enol from presence of which type of hydrogen is must?
a) Alpha
b) Beta
c) Gamma
d) Any position of hydrogen
Answer: a
Clarification: For Keto-Enol tautomerism, keto form should have α-hydrogen, as shown in below example.

7. Which of the following tautomeric form is more stable?

CH₃-CH=O   ↔   CH₂=C-OH 
      (I)                         (II)

a) I
b) II
c) Both are equally stable
d) Both are unstable
Answer: a
Clarification: The keto–enol tautomerization chemical equilibrium is highly thermodynamically driven, and at room temperature the equilibrium heavily favors the formation of the keto form.

8. What is the stability order for the following compounds?

a) I > II > III
b) II > I > III
c) III > II > I
d) I > III > II
Answer: b
Clarification: The keto–enol tautomerization, less stable enolate ion will be formed faster as it is thermodynamically stable, so III is more stable than I.
II is most sable as it is keto form.

9. What is the stability order for the following compounds?

a) I > II > III
b) II > I > III
c) III > II > I
d) I > III > II
Answer: b
Clarification: The keto–enol tautomerization, less stable enolate ion will be formed faster as it is thermodynamically stable, so III is more stable than I.
II is most sable as it is keto form.

10. Which of the following structure can show tautomerism?

a) I
b) II
c) III
d) I and III
Answer: b
Clarification: Quinone has an α-hydrogen; however, it is a vinylic hydrogen (hydrogen attached to a carbon atom which is sp2 hybridized), it is very difficult to abstract such a hydrogen, and hence it becomes very less acidic. Therefore, no abstraction of α-hydrogen from it, by a base, hence, it does not show tautomerism, same is for III.

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